1. Field of the Invention
The present invention relates to an apparatus for controlling a power supply.
2. Description of the Related Art
Many electronic devices, such as a laptop type personal computer and a word processor, are operated by an AC adapter or a rechargeable built-in battery, or on both.
Such an electronic device has a capacitor of a large capacitance provided on a power line from a power supply to supply a voltage to individual components of the electric device. When the power supply is deactivated, therefore, the power supply voltage on the power line will become 0 V only after a predetermined period of time elapses.
For instance, in the case where a capacitor of about 3000 .mu.F is provided on the power line from the power supply that provides a voltage of +5 V and a load at the time of power turn off is about 100 .OMEGA., it takes approximately 480 ms for the power supply voltage to become 1 V after the power supply is deactivated.
When the power supply is activated immediately after the electronic device is powered off, since the power supply is activated with a state that the power supply voltage on the power line has insufficiently been reduced, the following problems would arise.
With regard to a hard disk drive, for example, when the power supply is activated immediately after the turn off of the power supply, it takes time for the motor used in the hard disk drive to operate properly. In other words, when the power supply is deactivated while the motor is running, the rotor of the motor keeps rotating for a while due to the inertia. When the power supply is activated while the motor is rotating under this condition, since the hard disk drive operates to rotate the rotor by driving the motor again after rotation of the rotor completely stops. This takes time for the motor to operate properly.
After the power supply is activated, a power supply circuit in the power supply normally performs such control as to gradually increase the power supply voltage, or soft-start control. FIG. 1 shows a diagram of a circuit used to execute the soft-start control. Referring to FIG. 1, as the terminal voltage of a terminal DTC of a switch regulator 1 used to determine the maximum value of the switching duty becomes lower, the maximum value of the switching duty increases. When the power supply is activated, a low voltage is supplied from a terminal REF, and the terminal voltage at the terminal DTC decreases as the charging of the capacitor 2 progresses. The time necessary to sufficiently decrease the terminal voltage at the terminal DTC is determined by the time constant of the capacitor 2 and a resistor 3. When the power supply is reactivated immediately after its deactivation, since the capacitor 2 is not sufficiently discharged, the proper soft-start control cannot be executed. If the soft-start control is not executed at the time the power supply is activated, the charging of the capacitor provided on the power line in the power supply circuit causes a large current to flow through a switching FET (Field Effect Transistor), thereby damaging the FET sometimes.
DC-driven electronic devices having a capacitor provided on the power line in the power supply circuit has the above-described problems when the power supply is reactivated immediately after the power supply is deactivated.
For such an electronic device as a personal computer having a resume function, when the power supply is activated, the device carries out a data restoring process to restore the system to the state immediately before the previous deactivation of the power supply, or a resume process. When the power supply is deactivated during execution of this resume process, part of data group being restored may be destroyed. If the power supply is reactivated and the resume process is performed in such a case, only the data group without a destroyed part can be restored. In the case of a personal computer using 80386SL CPU recently developed by Intel corporation, when the power supply is deactivated during execution of the resume process, this CPU is likely to run away.
In view of the above, there is a demand for electronic devices which can properly operate even when the power supply is reactivated immediately after its deactivation, or even when the power supply is deactivated during execution of a resume process.